1. Field of the Invention
The present invention relates to a drum brake used for braking cars and specifically, to the assembling operation of such a drum brake.
2. Description of the Related Art
A drum brake such as shown in FIG. 5 (Prior Art) has been known for use in braking cars.
Such a drum brake comprises: a brake drum (not shown) that rotates along with the wheel; a backing plate 1 fixed on a suspension system; a pair of brake shoes 3a and 3b that are displaceably supported by the backing plate 1 and pressed onto the inner peripheral surface of the brake drum by being expanded by a wheel cylinder 2 fixed on the backing plate 1; a parking lever 4, one end of which (upper end as viewed in FIG. 5 (Prior Art)) is pivotally supported by one brake shoe 3b (right as viewed in FIG. 5 (Prior Art)) and through a pivot pin 8 and the other end of which (lower end as viewed in FIG. 5 (Prior Art)) is connected to a cable (not shown) for operating the parking brake mechanism; and a strut 6, one end of which (right end as viewed in FIG. 5 (Prior Art)) is abutted against the middle of the parking lever 4 and the other end of which (left end as viewed in FIG. 5 (Prior Art)) is abutted against the other brake shoe 3a (left as viewed in FIG. 5 (Prior Art)). The strut 6 has a built-in automatic gap adjusting device for adjusting the gap between the outer peripheral surfaces of the respective brake shoes 3a and 3b and the inner peripheral surface of the brake drum as the linings 5 of the brake shoes 3a and 3b wear.
To effect braking by the drum brake thus constructed, the distance between the single ends (upper end as viewed in FIG. 5 (Prior Art)) of the pair of brake shoes 3a and 3b are expanded by the wheel cylinder 2 arranged on the backing plate 1. Both brake shoes 3a and 3b are displaced toward the outside of the backing plate 1, pivoting around an anchor 7 fixed on the backing plate 1. The outer peripheral surfaces of the linings 5 of the brake shoes 3a and 3b are pressed against the inner peripheral surface of the brake drum, thereby causing a braking force to be produced by the friction between these surfaces. To release the braking force, the supply of hydraulic oil to the wheel cylinder 2 is interrupted and the brake shoes 3a and 3b are returned to their original position by a return spring 9, thereby disengaging the outer peripheral surfaces of the linings 5 from the inner peripheral surface of the brake drum.
To operate the parking brake, the other end (lower end as viewed in FIG. 5 (Prior Art)) of the parking lever 4 is pulled toward the left by the cable, thereby causing the parking lever 4 to pivot clockwise around the pivot pin 8 as viewed in FIG. 5 (Prior Art). As a result, the strut 6, one end of which abuts against the middle of the parking lever 4, is pushed toward the left as viewed in FIG. 5 (Prior Art), thereby pushing the brake shoe 3a against which the other end of the strut 6 abuts, toward the outside, thus pushing the outer peripheral surface of the lining 5 of brake shoe 3a against the inner peripheral surface of the brake drum.
At the same time, as a reaction, the pivot pin 8 supporting one end of the parking lever 4 is displaced toward the outside causing the brake shoe 3b on which the pivot pin 8 is arranged to be pushed outward, thus pushing the outer peripheral surface of the lining 5 of brake shoe 3b against the inner peripheral surface of the brake drum. As a result, the outer peripheral surfaces of the linings 5 of both brake shoes 3a and 3b are pressed against the inner peripheral surface of the brake drum, thereby producing a braking force by the friction between the surfaces.
In a case where the linings 5 are worn as a result of repeated braking operations and the gap between the outer peripheral surfaces of the linings 5 and the inner peripheral surface of the brake drum is increased, the automatic gap adjusting device built into strut 6 causes strut 6 to expand, thereby maintaining the gap within a prescribed range.
In a drum brake thus constructed and operated, both the linings 5 and the inner peripheral surface of the brake drum (in a far smaller degree than the linings) wear as a result of the repeated braking operations. The wear is located only where the linings 5 contact the drum and takes the form of a so-called "stepped wear," in which a stepped groove is observed with respect to the other portions of the inner peripheral surface of the brake drum. In a case where such stepped wear is present and becomes larger than a proper gap between the outer peripheral surfaces of the linings 5 and the inner peripheral surface of the brake drum, the engagement of the side end of each of the linings 5 with the step prevents the removal of the brake drum, thereby preventing the brake shoes 3a and 3b from being replaced.
For this reason, the conventional brake drum is so constructed that it can be removed even when the stepped wear is present by turning the parking lever 4 in a direction opposite to that in the braking operation (counterclockwise as viewed in FIG. 5 (Prior Art)) through an operation performed outside the backing plate 1. As shown in FIG. 6 (Prior Art), a stopper strip 10 is fixed at a location which is part of the backing plate 1 and through which the other end (lower end as viewed in FIG. 5 (Prior Art)) of the parking lever 4 passes. Normally, the amount of rotation of the parking lever 4 in the direction of releasing the braking force is restricted by this stopper strip 10. When removing the brake drum, this stopper strip 10 is removed, thereby increasing the gap between the inner peripheral surface of the brake drum and the outer peripheral surfaces of the linings 5.
To remove the brake drum so as to replace the brake shoes 3a and 3b, a pin 11 supporting the stopper strip 10 is removed from outside the backing plate 1. Then the stopper strip 10 is removed from the backing plate 1. As a result, the parking lever 4 is turned around the pivot pin 8 counterclockwise as viewed in FIG. 5 (Prior Art). The force expanding the pair of brake shoes 3a and 3b is released through the strut 6. The gap between the outer peripheral surfaces of the linings 5 and the inner peripheral surface of the brake drum increases in excess of the proper range so that the brake drum can be removed even if stepped wear is present on the inner peripheral surface of the brake drum.
However, the conventional drum brake thus constructed and operated presents several shortcomings. Since the stopper strip 10 and the backing plate 1 are formed in separate pieces, there is a risk of forgetting to mount the stopper strip 10. Once the strut 6 has been expanded without the stopper strip 10 (i.e., if the gap between the linings 5 and the brake drum has been adjusted), the brake drum can no longer be removed.
To assemble the drum brake, on the other hand, the other end (lower end as viewed in FIG. 5 (Prior Art)) of the parking lever 4 is located further left than the stopper strip 10 as viewed in FIG. 6 (Prior Art), and it is necessary to pull the other end of the parking lever 4 with a cable. Once the drum brake has been assembled with the other end being mistakenly located on the side opposite the stopper strip 10 (right as viewed in FIG. 6 (Prior Art)), the parking lever 4 can no longer be turned by the cable, thereby making the parking brake inoperative.
Further, to install the brake drum, the gap between the inner peripheral surface of the brake drum and the outer peripheral surfaces of the linings 5 is increased and the strut 6 is contracted so that the brake drum can readily be installed. However, the automatic gap adjusting device that is built into the strut 6 makes only a few such gap adjustment at one braking operation (the amount of expansion of the strut 6 made every braking operation is very small). Thus, after the installation of the brake drum with the gap between the inner peripheral surface of the brake drum and the outer peripheral surfaces of the linings 5 being sufficiently large, the braking operation (the operation of pulling the cable and turning the parking lever 4) must be repeated many times to have the gap between the linings 5 and the brake drum properly adjusted, thereby entailing cumbersome work.
Although Unexamined Japanese Utility Model Application (OPI) No. 96825/1982 (the term OPI as used herein means "Unexamined Published Application") discloses a device that properly adjusts the gap with only one braking operation after the installation of the brake drum, its structure is relatively complicated, making it not readily practical.
The drum brake according to the present invention overcomes the above-described shortcomings.